Flexible polyolefin resins are widely used as raw materials for films and the like as a substitute for a flexible vinyl chloride resin with a large environmental load.
Since a flexible polyolefin resin contains a large amount of low molecular weight components by its nature, the surface of granules made of the resin may exhibit tackiness. Therefore, when a polymerized flexible polyolefin resin is granulated into granules with a size easy to handle, there has been a problem that granules tend to adhere among themselves and form lumps (blocks). Furthermore, refusion of granules is easily conducted after granulation.
As the method for granulating a flexible polyolefin resin, for example, a method of extruding a resin from an extrusion die with a plurality of outlets facing downward toward a cooling water pool, cutting the extruded resin into pellets by means of a rotating cutter under the outlets, and spraying cooling water containing a releasing agent from below has been disclosed (see Patent document 1, for example).
However, this method requires use of a specific resin composition and application of a releasing agent to the surface of the granules. In addition, since pellets float on water surface in cooling process after granulation, a cooling efficiency is impaired.
The inventors of the present invention have found that a tacky feeling of granules of a flexible polyolefin resin can be reduced by melting the resin, and granulating the resin after stirring and kneading the molten resin while cooling the molten resin to a temperature below the melting point. The inventors then filed a patent application directing to the above finding (Patent document 2).
Patent Document 1: JP-A-H7-88839
Patent Document 2: JP-A-2005-179556
However, the method of Patent document 2 cools the polymerized resins and thereafter heats the resins again for melting, which results in low productivity. The method is thus desired to be further improved. In addition, the method of Patent document 2 requires expensive facilities such as a kneader to stir and knead a molten resin while cooling the molten resin after melting the resin. Furthermore, since pellets float in a cooling water pool in some granulation methods, the cooling efficiency of the method of Patent document 2 is impaired. Therefore, further improvement has been demanded for higher granule productivity.
In view of the above problems, an object of the invention is to provide a method for efficiently granulating a flexible polyolefin resin.
An object of the invention is to provide an efficient granulating method without refusion of granules after granulation.